Design Criteria for Irrigation Canals - Sediment Transport in Irrigation Canals: A New Approach - page 73

Transport of bed load
For canal networks that transport some material as bed load, it is possible
to establish a comparable criterion as for the transport of suspended load.
Irrigation canals that carry substantial sediment loads cannot be treated
either by the rational method or in this way.
Bed load transport will mainly occur on or above the canal bottom;
almost no transport will occur on the banks. The bed load might be some
sediment with a diameter larger than 50 to 70 * 10 − 3 mm. The amount
of bed load transport mainly depends on the shear velocity v ∗ =

( gyS o ) 0 . 5 .
The continuous transport of bed load material can be described by one of
the various sediment transport formulae, for example Engelund Hansen,
Ackers-White, Brownlie, etc.
The relative transport capacity is:

y 3 S o

T
Q ∝

b ∗ ∗

(4.16)

b ∗ y ∗ y x ∗ S o

T
Q ∝

y 2 − x S 3 − z
o

(4.17)

where:
T / Q = relative transport capacity

T = sediment transport load (m 3 /s)

Q = discharge (m 3 /s)

b = bottom width (m)

y = water depth (m)

S o = bottom slope (m/m)

x , z = exponents depending on the choice of the water flow equations,

for instance Manning, Strickler, de Chézy or Lacey. For example,
the exponents for the regime theory are x = 0.75 and z = 0.5;

for the Strickler equation they are x = 3/2 and z = 0.5.

y 2 − x
S 3 − o should either be constant or not decrease in the downstream direction
of the irrigation network. For x

To prevent sedimentation the relative transport capacity T/Q =

0.5 the relationship T/Q
is proportional with y 0 . 5 S o . Also with other flow equations (Manning,
Strickler or de Chézy) and sediment transport predictors (for instance,
Engelund-Hansen or Einstein-Brown), the relative sediment transport
capacity proves to be almost proportional with y 0 . 5 S o .
To prevent erosion, the boundary shear stress or the shear veloc-
ity should not increase, but should remain constant or decrease in a
downstream direction.
The shear velocity follows from:

= 0.75 and z =

gyS o

v ∗ = (4.18)